The present invention relates to aircraft fire extinguisher systems, and is more particularly concerned with monitors for checking continuity in a resistive explosive squib in a fire extinguisher.
Aircraft fire extinguishers are discharged by supplying a large current to an explosive, resistive squib attached to the fire extinguisher bottle. Firing of the squib causes the extinguisher substance in the bottle to be released. Because the fire extinguisher system must be of high integrity, it is usual to test the continuity of the circuit including the squibs at the start of each flight leg. The conventional way of testing this continuity is to supply to the circuit a low current insufficient to fire the squib or cause deterioration. If the current flows to ground, there is assumed to be continuity through the squib to ground. This test, however, is unable to distinguish between a correctly operating fire extinguisher circuit and one where the circuit is shorted to ground before the squib. Furthermore, the conventional way of testing is not able to identify a squib which is out of tolerance unless this is excessive.
One way of overcoming these difficulties would be to measure the resistance of the squib. This, however, presents problems in that the potential of the ground point to which the squib is attached is unknown because it may be at some distance. The resistance could be measured if additional wiring were utilized but this is undesirable in aircraft applications because of the increased weight and complexity.